Dairy Digesters Fight Climate Change, Benefit Farms

While many consider dairy manure digesters as inherently unattractive projects—they are loud, expensive, and manure management is probably not something you want to think about while enjoying your ice cream—they also offer an abundance of benefits, including renewable energy. Fortunately, there are people that recognize the problems with the current U.S. manure management systems, and work to bring more of these critical projects to fruition, so the rest of us don’t have to think too hard about manure.

I had the pleasure of attending the Washington State University (WSU) Anaerobic Digestion Systems Field Day in June, where interested parties gathered to share knowledge around the issues of managing a lot of manure. I work on dairy digester projects, primarily because of their greenhouse gas benefits, so this event was a great opportunity to get out in the field and learn more about the many alternative benefits, or “co-benefits,” that make manure digesters important for modern dairy practices.

The Dairy

Dairies in Washington State produce a combined 7 million tons of manure per year from 415 operations. Common practice is for manure to be flushed with water and stored in open lagoons where it vents methane—a greenhouse gas 72 times more powerful than carbon dioxide—into the atmosphere and generally stinks up the place. The untreated manure is later applied to land, which can cause problems with over-fertilization, nutrient runoff, and ground water pollution.

The field tour took us to Edaleen Dairy in Lynden, Washington, which milks about 1,700 cows a year and produces a lot of manure. The dairy installed a manure digester three years ago, and has since been selling power to the local utility, as well as offsets into the California carbon market.

One of the more memorable presentations of the day was with the dairy operator. He explained that when pitching the digester idea to the dairy owner, he knew if he presented a list of economic and environmental benefits that yield greater revenues, he would get “punched.” So, while the primary revenue sources that make a digester possible are the sale of energy and environmental credits, the bottom line for this dairy owner was whether or not the change was better for the cows. Surprisingly, it turns out that it is!

Dairy Digester Products

Now let’s talk poop. Adjacent to the digester was the primary solids separator. After the manure liquid has been processed through a digester, the solids can be separated out and used as a sanitary animal bedding as the digester heat kills over 99% of pathogens. This clean bedding leads to cleaner cows, fewer sick animals and reduced use of medication. Not to mention the costs savings from a reduced need to purchase animal bedding.

After most of the solids have been separated out, the liquid effluent may be applied to land as a fertilizer, now in a safer form. Edaleen dairy spread their effluent on their surrounding corn fields. As part of the tour, the group reviewed emerging technologies that are being developed to produce additional products that could be useful on and off the dairy.

If a dairy chooses not to use the fibrous solids as animal bedding, then pyrolysis—the application of heat in the absence of oxygen—can produce biochar. This renewable coal-like substance not only acts as a fertilizer in soil, but is also a great carbon sink as the carbon rich substance doesn’t break down for hundreds of years.

Another option is advanced solids separation technology that can isolate nutrients from the manure, such as phosphorus and nitrogen. Other nutrient recovery technologies, such as struvite crystallization, can recover additional nitrogen and phosphorus. This isolation of nutrients allows for more targeted application in fields and reduces the chance of nutrient waste or pollution. Also, water treatment technologies are now advanced enough to produce quality water for reuse on the farm.

Many of these technologies remain prohibitively expensive at face value, but WSU is working with local farmers to research additional cost savings and revenue sources of the manure-derived products. Legislation is being introduced at the federal level to create tax incentives for nutrient recovery systems as well.

The Raspberry Farm

The second site visit of the tour was to a nearby raspberry farm that is experimenting with the application of cow waste products. The farmer acknowledged that after decades of traditional monoculture berry production—the cultivation of a single crop in a given area—his berry yields have been declining. However, he’s committed to changing his practices to make his farm more sustainable. As part of these efforts, he commissioned a custom-built manure application device that he tows though his berry fields to directly apply liquid manure to the raspberry stalks early in the growing season. This is the second year he’s tried direct manure applications in addition to traditional methods, so the impact of the change has yet to be determined.

WSU is also working with the farmer to test a suite of manure-derived fertilizer products at several plots around the farm in an effort known as “dairies to berries.” They are not only looking at change in fruit yields compared to untreated manure and inorganic fertilizers, but also how much of the nutrients are leaching into soil beyond the reach of the raspberry roots and into the environment. This data will help ensure that the increase in soil health does not come at the cost of environmental contamination, which translates to increased long-term revenue and less risk for the farmer.

The research is ongoing, in fact, several students moved about the farm collecting sample data during the course of the tour. New fertilizers require a lot of testing before they are widely accepted, but the research is promising that dairy manure-derived fertilizers will be as effective as traditional, less sustainable fertilizers, in addition to carrying less risk for the environment.

Sustainable Manure Starts with a Digester

The primary benefits of anaerobic manure digestion are having a consistent source of renewable energy and environmental credits from the reduction of greenhouse gas emissions, but there are many other co-benefits available now and under development, such as odor control, safer animal bedding, and improved soil nutrient management. However, most dairies lack a manure digester due to the large upfront costs and construction being dependent on grants. While costs are expected to come down over time, and revenue streams will increase as the co-benefits are better understood, increased sustainability isn’t possible if farmers continue to manage their manure in open lagoons.

Given the many co-benefits of operating a digester, it may not be surprising that one-third of The Climate Trust’s offsets under contract since 2010 have been in the biogas sector. While The Climate Trust will continue investing in these high impact emission reductions projects, there is still much to be done in this sector. The next time you crave ice cream this summer I encourage you think about where it comes from, and strive to support sustainable farmers that improve the health of their animals and soils for generations to come.